I guess that the machine can accurately calculate your energy output, if it knows your weight, as that's a fairly simple calculation for it to do

It'll then need to come up with an efficiency quotient to give you an energy consumption. This is where the fudge comes in; I'd like to think that the manufacturer's have had dozens of test subjects connected up to breathing equipment to do some rig tests but I suspect that's not true! Perhaps there's some industry wide algorithms for this kind of stuff, in which it'll be more accurate if it asks for more data (height, weight, sex, build etc.)